Hammer mills have long been used for grinding or comminution of various materials. A typical hammer mill comprises a rotor assembly mounted on a rotor shaft inside a housing. A rotor assembly 1100 is illustrated at rest in FIGS. 11 and 13 of Plumb et al. U.S. Pat. No. 8,104,177, which patent is incorporated herein by reference. A material inlet is generally located at the top of the housing with one or more material outlets located near the bottom of the housing. As shown in FIGS. 11 to 13 of the Plumb et al. '177 patent, the rotor assembly 1100 includes a drive shaft and rows of hammers 1400, as illustrated in FIG. 14 of the Plumb et al. '177 patent. The hammers 1400 are pivotally connected to the rotor 1100 by a steel hammer rod or pin. The hammers are normally flat steel blades or bars, as illustrated in FIGS. 11 to 14. The hammers extend out substantially radially from the hammer rods due to inertia when the hammermill is (rotating) in operation, as illustrated in FIG. 12. The rotor assembly 1100 is mounted inside a housing, known by those skilled in the hammer art as a grinding or working chamber. In a reversible hammer mill, this grinding chamber comprises a cutting plate mounted on either side of the material inlet. Reversible hammer mills are capable of rotation in either direction, a feature providing increased life for the hammers 1400, cutting plates, and screen plates. Sedberry, U.S. Pat. No. 1,433,042 shows reversible hammers (FIG. 5 and lines 38 to 41 on page 3). Williams, U.S. Pat. No. 1,760,097 shows reversible hammers (lines 12 to 17 on page 1). Plumb et al., U.S. Pat. No. 8,104,177 teaches reversible hammer mills are well known in the hammer art (lines 35 to 38 of col. 1).
Present-day cutting plates comprise an upper, linear section, and do not allow particles to escape. Downstream of the cutting plate, the interior of the working chamber is defined by curved screen plates. The screen opening diameter is selected to match the desired final particle size of the material being comminuted. Particles less than or equal to the desired size exit the chamber though the screens, while material greater than the desired size are further reduced by the rotating hammers 1400.
Numerous industries rely on hammer mill grinders or impact grinders to reduce material to a smaller size. For example, hammer mills are often used to process forestry products, agricultural products, minerals, and materials for recycling. Specific examples of materials processed by hammer mills include corn, grains, animal feeds, pet food, feed ingredients, mulch, wood, hay, plastics, and dried distillery grains. Hammer mills heretofore have long been employed to effect size reduction of such diverse materials such as scrap metal including auto bodies, paper, animal and human feed, and anything else that need to be reduced in size.
Standard hammers, when grinding or comminuting materials, impact the product to be pulverized to create a smaller size particle. This impact forces material against a perforated screen area and cuts and sizes the product. Inside the typical hammer mill, numerous forces act. Forces exist at the contact end of the hammer, where the hammer impacts the material being comminuted. Present inside the mill are axial forces, parallel to the rotor shaft, exerted on the sides of the hammers by the materials fed into and passing through the mill. The combination of these forces on the hammers causes elongation of the hammer rod hole, decreased hammer life, and eventual failure of the hammer. Hammer failure results in costly shutdown of the mill and hammer replacement time. Accordingly, there is a need for a hammer design to compensate for impact loading, side loading, wear, and hammer rod hole elongation. Plumb et al., U.S. Pat. Nos. 8,104,177 and 8,342,435, which are incorporated herein by reference, illustrate hammers having a circular bearing race in the connection end of the hammer body, and received therein a circular bearing having a circular inner surface or race which receives a hammer rod (FIG. 6). The combination permits the hammer to move in the rotary, as well as in the axial direction.
All conventional free swinging hammers have a hole diameter that is larger than the rod diameter it is installed upon. The typical diameter oversize is 0.030″; when the hammer is loaded the bearing surface between the rod and the hammer hole initially is a just a tangent point as the rod radius and the hole radius are not equal; through loading the hole deforms and the radiuses become the same. Deformation in a hardened material however is a leading cause of hammer failures. Having an elongated, oval, or egg shape hole allows the hammer hole to be oversized in portions of the hole for installation but upon loading the upper portion of the hole will have the same radius as the rod it is installed upon. This increases the surface area of support in the rod hole in end minimizing stress on the hammer to rod connection point and mitigating deformation. As hammers are in operation the product being milled can become entrapped between the hammer and the rod causing the hammers to stick and not free swing upon start-up and shutdown causing vibration in the mill. Increasing the hole size can mitigate this issue but drastically increases the point load on the hammer and rod. Having an inner oval, egg-shaped or elongated hole allows the hammer to have a matched radius contact point and a larger void at the bottom of the rod, increasing the area between the rod and the hammer allowing the milled material to escape to mitigate hammer sticking issues.
The Plumb et al. patents also show a well known expedient of providing hardened material 22 added to edge portions of the contact end of the hammer, preferably by welding, to increase hammer life. See also Newell, U.S. Pat. No. 3,482,789 (lines 12 to 18 of column 4); Kachik, U.S. Pat. No. 4,856,170 (lines 55 to 68 of column 8); and, Lowry U.S. Pat. No. 4,129,262. Welding of tungsten carbide onto the contact surfaces of hammers is well known in the art, as illustrated in the Young U.S. Pat. No. 7,140,569 (lines 9 to 17 of column 3); Young, U.S. Pat. No. 8,033,490 (lines 28 to 36 of column 9); Balvanz, U.S. Pat. No. 6,419,173 (lines 11 to 16 of column 3); Rogers, U.S. Pat. No. 2,647,695 (lines 37 to 41 of column 2); Mankoff, U.S. Pat. No. 2,763,439 (lines 26 to 31 of column 1); and, Eilers, U.S. Pat. No. 3,045,934 (lines 44 to 54 of column 2). Hammers produced by Jacobs Corporation, as illustrated in Ronfeldt et al., U.S. Pat. No. 7,419,109, included hardened material, such as tungsten carbide, welded on the contact edges of hammer mill hammers.
The hammers of the preferred embodiments herein include scalloped portions on the leading and trailing edges of the contact ends of the hammers. These scalloped edge portions of the contact end of the hammers may include tungsten carbide welded on the surface thereof. Hammers with triangular and/or square notches in the contact side edges of hammers are known in the prior art. See Williams, U.S. Pat. No. 1,760,097; Iglehart, U.S. Pat. No. 1,827,986; Alfred, U.S. Pat. No. 1,829,325; and, Jensen, U.S. Pat. No. 1,954,175. Tankersley, U.S. Pat. No. 2,237,510, shows grooves in the sides of a hammer extending from the contact end toward the connection end. Williams, U.S. Pat. No. 5,002,233, shows a circular cut in the contact edges of a hammer.
Manufacturing methods utilizing casting, forging, heat treatment and rolled steel are well known in the art to improve the life and functionality of the hammer. Nielsen, U.S. Pat. No. 1,889,129, teaches casting hammermill hammers (lines 35 to 40 on page 1). Ball, U.S. Pat. No. 2,602,597, teaches case hardening hammers parts, a heat treatment, to increase wear life (lines 51 to 56 on page 1). Rogers, U.S. Pat. No. 5,377,919, teaches heat treating hammers (lines 30 to 33 of col. 4). Plumb et al., U.S. Pat. No. 8,104,177 teaches manufacturing methods utilizing forging, rolling and casting are well known in the hammer art to improve grinding characteristics, and life and functionality of the hammers. Young, U.S. Pat. No. 7,140,569, teaches forging (lines 20 to 27 of col. 3). Young, U.S. Pat. No. 8,033,490, teaches forging is preferred because it produces a much strong hammer than casting, but that other methods such as casting, rolling, stamping, machining and welding are known to those of ordinary skill in the art (lines 7 to 17 of col. 13). Young, '490, also teaches a multiple-blade hammer may be heat treated for hardness (line 5 to 13 of col. 15).
The hammers of the present invention have a non-circular, elongated, oval or egg-shaped hole in the inner bearing race, on the inside of the circular bearing received on the inside of the outer bearing race in the connection end of the hammer body. Hellmich, U.S. Pat. No. 5,598,981, shows hammers with a circular hammer rod hole, wherein the circular hammer rod hole is significantly larger in diameter than the hammer rod. The hammer rod in Hellmich is provided with a cam shaped portion such that upon rotation of the hammer rod the hammer length is adjusted radially to compensate for wear at the contact end of the hammer. Jacobson et al., U.S. Pat. No. 3,598,008, shows a hammer wherein the hammer rod hole consists of two overlapping circular sections, a smaller diameter circular section situated toward the end of the hammer for receiving a hammer rod of substantially the same diameter, and a substantially larger diameter circular section overlapping the smaller diameter circular section and situated toward the contact end of the hammer. Benson, U.S. Pat. No. 2,886,117, shows hammers affixed to plates P by pivot pins 27. The plates have two overlapping circular holes for receiving the pivot pins, the radially outermost circular hole having a smaller diameter for receiving the pivot pins, and the innermost circular hole having a larger diameter. Sheppard, Jr., U.S. Pat. No. 1,803,148, shows a two-piece hammer having a first portion including a circular rod hole at the connection end thereof, and a second end portion thereof having plural arms extending radially therefrom. The ends of the arms have holes for receiving a bolt there through, which bolt holds plural hammer heads. The hole in the hammer heads through which the bolt passes are elongated. Potwin, U.S. Pat. No. 4,142,687, similarly shows two piece hammers, the connection piece having a first portion with a circular rod hole for receiving a hammer rod and a second portion having a slot 32 for receiving securing cross pin 42 and spacers 54. Hightower, U.S. Pat. No. 3,844,494 shows a circular hole having a hardened bushing with a circular inner hole therein for receiving a hammer rod there through (lines 60-66 of col. 3).
All of the prior art patents referred to in this document are hereby incorporated by reference herein in their entirety.